1. Field of the Invention
This invention relates to a filter of depth layer type which can remove impurities from a fluid with very high reliability in spite of a simple structure.
2. Description of the prior Art
A filter body of a filter of depth layer type has such a density distribution that the density is lowest in an area of its inlet side where a fluid such as oil to be filtered flows into the filter, and the density is progressively increased toward an area of its outlet side where the oil flows out from the filter.
Because of such a density distribution, the filter of depth layer type can arrest a large amount of large and small impurities at a low pressure with least possibility of clogging. A filter of so-called roll paper type is known as an example of prior art filters of this kind. In the filter of roll paper type, a filtering material is wound into the form of a roll, and one of the axial ends of the roll is radially most compressed to have a highest density so as to provide a density distribution in which the density progressively decreases from that end toward the other end of the roll. A fluid such as oil is supplied from the large-diameter side toward the small-diameter side of the roll, that is, from the low-density inlet side toward the high-density outlet side of the roll to remove impurities from the oil.
A filter of herringbone twill type for filtering a fluid such as oil is also known as another example of prior art filters of this kind. The filter of twill type includes a filter body made by winding a braid-like filtering material in a twill pattern around a hollow cylindrical core member. The manner of winding is such that the filtering material is initially closely wound under a strong tension on the outer peripheral surface of the core member and is then progressively coarsely wound under a progressively lower tension. That is, the filter body of the filter of twill type has a density distribution in which the density becomes progressively higher radially inward.
However, in the filter of roll paper type described above, creases tend to be produced on the surface of the filter body during compression of the filtering material in the radial direction, and, when these surface creases are produced as a result of strong compression, undesirable paths of oil are formed in the filter body through which oil flows directly without being filtered. Therefore, the filter of roll paper type has had the problem that the desired delicate density distribution cannot be provided in the filter body due to the impossibility of strong compression of the filtering material, and the impurity arresting capability cannot be improved. Also, in the case of the filter of herring bone twill type, the total length of the filtering material wound in the twill pattern around the core member is large, and, in addition, the filtering material must be wound while changing the tension and direction of winding to provide the radial density distribution described above.
Therefore, the filter of twill type has had the problem that the requirement of a great deal of labor and time results in a high cost, and uniformity of the properties of products is difficult to attain.